1. Field of the Invention
The invention relates to a communication system which can communicate even at the time of a power failure.
2. Related Background Art
Hitherto, there is a communication system in which a circuit of a single telephone is provided in an exclusive-use telephone which is connected to a main apparatus by a wire and, when a power failure occurs, such a circuit is directly connected to an external line, thereby enabling a transmission and a reception to be executed in only such an exclusive-use telephone. In case of using the above construction, the communication system can be cheaply realized as compared with a construction such that the whole system is operated by a power source for a power failure.
A dial button of the exclusive-use telephone corresponding to a power failure having therein the circuit of the single telephone needs two circuits of a signal line to scan from a CPU at the time of a current supply and a signal line to scan from a dialer (or a dialer IC) at the time of a power failure.
With respect to such two circuits of the signal lines for scanning the dial button at the time of the current supply and the power failure, it is necessary to assure an insulation state so as to prevent that the system is broken by an induction lightning from the external line. A printed circuit board and a switch part to realize such two circuits of the signal lines for scanning are expensive.